A micromirror array in which multiple “unit optical elements each reflecting light by means of one or more mirror surfaces” are disposed on a substrate (base) constituting an element surface of an optical element has been developed as an image-forming optical element which image-forms a three-dimensional or two-dimensional object, image and the like. In particular, a micromirror array including a large number of recessed unit optical elements or protruding unit optical elements arranged in an array and each having “two mirror surfaces orthogonal to each other” (a corner reflector) disposed at right angles or at an angle close to the right angles to this substrate has received attention in recent years because it is relatively simple in structure and it is expected to reduce manufacturing costs (with reference to Patent Literature 1).
Examples of the aforementioned micromirror array are shown in FIGS. 12 and 13.
In a recessed type micromirror array 50 (hereinafter referred to simply as an “array” in some cases) shown in FIG. 12, a large number of minute holes 51 in the shape of a quadrangular tube (unit optical elements; the ratio of length, width and depth is approximately 1:1:1 in this example) are formed in one surface of a flat substrate 3 (element surface P) made of a transparent material so as to extend to the other surface thereof, and are arranged in a checkerboard pattern angled at 45 degrees with respect to an observer. At least two surfaces out of the four side surfaces (inner wall surfaces) of each of the unit optical elements (minute holes 51) are formed as mirror surfaces (light-reflective wall surfaces).
In a protruding type micromirror array 60 shown in FIG. 13, a large number of transparent minute protruding portions 61 in the shape of a quadrangular prism (unit optical elements; cubes having an approximately 1:1:1 ratio of width, depth and height in this example) are formed on one surface of a substrate 4 (element surface P) made of a transparent material, and are arranged in a checkerboard pattern angled at 45 degrees with respect to an observer. In the case of the aforementioned array 60, at least two surfaces out of the four side surfaces (wall surfaces) of each of the unit optical elements (minute protruding portions 61) are formed as mirror surfaces (light-reflective wall surfaces).
As shown in FIG. 14, when light incident on one surface (front or back) side of a micromirror array L of the aforementioned recessed type, protruding type or the like passes through the array L, this light (dash-double-dot lines) is reflected once from each of the two mirror surfaces on opposite sides of one corner K of each of the unit optical elements (twice in total). The light reflected twice (passing light) forms a mirror image (reversed image M′ indicated by a chain line) of an object M to be projected at a spatial position on the other surface side of each aforementioned array L (position symmetrical to the object M with respect to the plane of the element surface P).
An example of a method of producing a recessed type micromirror array as mentioned above which has hitherto been employed includes a method of reversely transferring the shape of the aforementioned unit optical elements by a nano imprint technique or an electroforming technique through the use of a metal mold (molding die) configured such that a large number of minute protruding portions complementary in shape to the recessed unit optical elements, respectively, are previously formed on a flat foundation (Patent Literature 1). An example of a method of producing a protruding type micromirror array which has been proposed includes a method of forming a large number of minute prisms arranged at predetermined spacings on a substrate by injection molding or hot press molding through the use of a metal mold (stamper) having a large number of minute cavities (recessed portions) complementary in shape to the protruding unit optical elements (Patent Literature 2).